Tio < Sub > 2 < / Sub > Nanostructures And Zno/tio < Sub > 2 < / Sub > Compound Preparation And Properties Of Nanostructures

Titanium dioxide (TiO2) is one of the extensively investigated transition-metal oxide materials due to its excellent optical, photocatalytic and electronic properties and hence its potential applications in various fields. TiO2is a semiconductor with a large band gap, a high dielectric constant as well as a high refractive index. TiO2can exist in three crystalline phases of brookite, anatase and rutile. The brookite and anatase structures are thermodynamically less stable while the rutile structure is very compact and the thermodynamically most stable phase at all temperatures. For optoelectronics applications, in particular, the rutile phase structure has been considered to be most suitable due to its high dielectric constant and refractive index. However, anatase TiO2has better properties of electron transportation. Furthermore, the properties of TiO2can be modified in nanostructured materials, which makes TiO2most promising in photocatalysis, chemical sensors. micro-electronic devices, optoelectronic devices, photoelectrochemical solar cells and so on, which is the starting point of this thesis.In this work, nanocrystalline TiO? thin films were deposited by electron cyclotron resonance plasma assisted reactive pulsed laser deposition (ECR-PLD) on non-heated substrates and then annealed. The as-deposited films show a structure of mixed anatase and rutile. A progressive improvement in crystallinity and transformation of TiO2phase occur in the deposited TiO2films depending on the annealing temperature. Annealing at about900℃in flowing N2> for30minutes results in a complete transition to the pure rutile nanocrystalline structure with high (110) orientation. The average size of the crystallites is estimated to be45nm. The prepared TiO2thin films show good transparency in the visible to near infrared region with small variations upon annealing while a red shift in absorption edge is observed due to high-temperature annealing.On the basis of successfully fabricating titanium dioxide thin films, we also investigated TiO2thin films doped with nitrogen. Mixture gas composed of oxygen and nitrogen discharged by ECR becomes oxygen and nitrogen plasma, in which Ti target is ablated bv laser beam. wherein the TiO2film is formed by the active oxygen’s reaction with ablated Ti and in-situ doped by active nitrogen during growing process. The results of TiO2thin filing doped with nitrogen confirm the presence of anatase and rutile phase and reveal that nitrogen atoms in the film are substitutionally introduced into some of the oxygen sites of TiO2lattice. As a result, a slight shift of the absorption edge to a lower energy in the optical transmission spectra has been observed.Vertically oriented nanotube arrays of TiO2have been fabricated by the anodization of pure titanium deposited by PLD (pulsed laser deposition) on silicon substrates. By investigating the process and conditions of anodization, we groped the optimal parameters for fabricating aligned TiO2nanotubes with uniform size and ordered shape. The TiO2nanotubes have an average diameter of approximately20-40nm.We successfully fabricated the ZnO/TiO2nanostructure composite films through two steps and attempted to modify the TiO2nanomaterials with ZnO in order to alter the charge transfer properties between TiO2and the surrounding environment and improve their photoelectrochemical properties. The TiO2nanotube arrays prepared by anodization method mainly present the anatase phase and the ZnO embedded in TiO2nanotubes fabricated by atomic layer deposition (ALD) show the hexagonal wurtzite phase. As an indirect band gap semiconductor. TiO2of anatase phase doesn’t illuminate, whereas the PL signal of ZnO/TiO2nanostructure composite film is a typical ZnO near-band-edge (NBE) ultraviolet emission, which ascribes a direct recombination of free excitons. Post-annealing could improve the intensity of ultraviolet PL. but it can also introduce oxygen vacancy at the same time, which leads to the appearance of visible broadband emission. Although annealing could enhance the transient photocurrent, we didn’t obtain the expected results of enhanceing the photoelectrochemical properties of ZnO/TiO2composite films.